This invention relates to a coding method for a digital signal applied to an encoder/decoder System which is adapted to carry out coding of an input signal such as a digital audio signal, etc. to transmit or receive a coded signal through a communication medium, or record or reproduce it through a recording medium to send such coded signal to a decoder to decode it to obtain a reproduced signal, a coder using such a coding method, a decoder for decoding an information signal coded by such a coding method, and a recording medium on which an information signal coded by such a coding method is recorded.
There are various efficient coding techniques for an audio signal or a speech signal, etc. For example, there can be enumerated a subband coding (SBC) which is a frequency band division system including no coding processing using blocks (signal sample blocks) in which an audio signal, etc. on the time base is divided into signal components in plural frequency bands without dividing such a signal into blocks (signal sample blocks) to encode them; a frequency band division system including coding processing using blocks (signal sample blocks), which is so called transform coding, adapted to transform a signal on the time base to a signal on the frequency base (spectrum transform) to divide it into signal components in plural frequency bands to encode such signal components in the respective bands; and the like.
Further, there has been proposed an efficient coding technique in which the above-described subband coding (SBC) and the transform coding are combined. In this case, for example, an approach is employed to carry out a band division by the subband coding (SBC) thereafter to spectrum-transform signals in the respective bands into signals (signal components) on the frequency base to implement coding to the spectrum-transformed signal components in the respective bands. As a filter for the band division, there is, e.g., so called a QMF filter. This QMF filter is described in 1976 R. E. Crochiere, Digital coding of speech in subbands, Bell Syst. Tech. J. , Vol. 55, No. 8 1976. In addition, in ICASSP 83, BOSTON, Polyphase Quadrature filters-A new subband coding technique, Joseph H. Rothweiler, a technique for carrying out band division by using filters of equal band width is described.
Here, as the above-described spectrum transform processing, e.g., there is a spectrum transform processing to divide an input audio signal into blocks (signal sample blocks) every predetermined unit time (frame) to carry out, in every block, Fast Fourier Transform (FFT), Discrete Cosine Transform (DCT), Modified DCT (NDCT), etc. to thereby transform signal components on the time base to signal components on the frequency base. Among these transforms, MDCT is described in ICASSP 1987, Subband/Transform Coding Using Filter Bank Designs Based on Time Domain Aliasing Cancellation, J. P. Princen, A. B. Bradley, Univ. of Surrey Royal Melbourne Inst. of Tech.
By quantizing signals (signal components) divided into bands by the filter(s) or the spectrum transform in this way, it is possible to control the band in which quantizing noise takes place. In this case, it is possible to carry out more efficient coding from the viewpoint of the hearing sense by utilizing the property of the masking effect. Further, if prior to implementation of quantization, normalization is carried out in the respective bands, e.g., by a maximum value of an absolute value of a signal component in each frequency band, more efficient coding can be carried out.
As a frequency division width for quantizing respective frequency components divided into frequency bands, there is carried out a band division in which, e.g., the hearing sense characteristic of the human being is taken into consideration. Namely, there are instances where an audio signal is divided into signal components in plural bands (e.g., 25 bands) having band widths adapted to become broader according as the frequency shifts to a higher frequency band side, which are generally called critical bands. Further, in encoding data in respective bands at this time, coding by a predetermined bit allocation is carried out in respective bands, or coding by an adaptive bit allocation is carried out in respective bands. For example, in encoding coefficient data obtained after they have undergone the MDCT processing by the above-mentioned bit allocation, coding is carried out by adaptive allocated bit numbers with respect to MDCT coefficient data in respective bands obtained by MDCT processing in respective blocks. As the bit allocation technique, the following two techniques are known.
In IEEE Transactions of Accoustics, Speech, and Signal Processing, vol. ASSP-25, No. 4, August 1977, bit allocation is carried out on the basis of the magnitudes of signals in respective bands. In accordance with this system, the quantizing noise spectrum becomes flat, and the noise energy becomes minimum. However, since the masking effect is not utilized from the viewpoint of the hearing sense, the actual noise sense is not optimum. In addition, in ICASSP 1980 The critical band coder-digital encoding of the perceptual requirements of the auditory system M. A. Kransner MIT, there is described a technique in which hearing sense masking is utilized to thereby obtain necessary signal-to-noise ratios in respective bands to carry out a fixed bit allocation.
However, with this technique, even in the case of measuring the characteristic by a sine wave input, since the bit allocation is fixed, the characteristic value would take a value which is not so good. To solve such problem, an efficient coder in which all bits which can be used for bit allocation are used in such a manner that they are divided into bits for the fixed bit allocation pattern determined in advance in respective small blocks and bits for the bit allocation depending upon the magnitudes of signals of respective blocks to allow the divisional ratio to be dependent upon a signal relevant to an input signal, and to allow the divisional ratio with respect to bits for the fixed bit allocation pattern to be greater according as the spectrum of the signal becomes more smooth is proposed in, e.g., U.S. patent application Ser. No. 07/924298 filed on Aug. 3, 1992, by this applicant, now abandoned, or U.S. Pat. No. 5,222,189 (Fielder).
In accordance with this method, in the case where the energy concentrates on a specific spectrum, like a sine wave input, many bits are allocated to a block including that spectrum, thereby making it possible to remarkably improve the entire signal-to-noise characteristic. In general, since the hearing sense of the human being is extremely sensitive to a signal having a sharp spectrum component, improvement of the signal-to-noise characteristic by using such method not only leads to an improvement in numeric values in measurement, but also is effective in improvement in the sound quality from the viewpoint of the hearing sense.
As a bit allocation method, a large number of methods are proposed in addition to the above. If the model relating to the hearing sense is caused to be finer and the ability of the coder is enhanced, more efficient coding from the standpoint of the hearing sense can be carried out,
In the system adapted for carrying out the fixed allocation, there is no necessity of transmitting bit allocation information from the coder to the decoder. Further, also in the system where bit allocation varies depending upon the frequency distribution of a signal, if normalization is carried out in respective bands to transmit their normalization coefficients to the decoder to have an ability to univocally determine bit allocation from those normalization coefficients, there is no necessity that the bit allocation information not be sent to the decoder.
However, with these methods, also in the case where the model relating to the hearing sense is caused to be finer in future so that the ability of the coder is enhanced, or the like, it was not possible to alter the bit allocation method. As a result, it was impossible to improve the sound quality or to carry out more efficient coding.
On the other hand, with the method of transmitting bit allocation information to the decoder, future improvements can be freely carried out, but many bits for transmitting bit allocation information itself were required, thus failing to carry out efficient coding.